Balancing Strength and Translucency of Zirconia

Objectives: Zirconia (Zr) has exceptional strength, however it is opaque. Recently, translucent Zr have been available. Objective: To compare the strength and translucency of the four classes of Zr (full-strength (FS), translucent (T), high-translucency (HT) and super high-translucency (SHT)).
Methods: 50 relevant publications were identified using the following databases: ScienceDirect, PubMed, MEDLINE, Academic Search Complete, Science & Technology Collection, and EBSCOhost. 31 Zr products from 13 different manufacturers were identified and organized into four classes based on their chemical composition. Their strength and translucency were compared amongst each other and with lithium disilicate (LD; comparison control).
Results: Seven Zr products were FS, 10 were T, 3 were HT, and 11 were SHT. Various mechanical (3-point (3P), 4-point (4P) and biaxial (Bi) flexural tests) and translucency (contrast ratio (CR), transparency parameter (TP) and translucency transmittance (Tt)) tests were used. All Zr groups were significantly stronger than LD, and SHT Zr was the weakest among the Zr groups. Bi test shows a decrease in strength from FS to SHT Zr, but 3P and 4P tests show that only the SHT Zr group is significantly weaker. TP results show that all Zr groups are less translucent than the LD group, and translucency increased from FS to SHT. However, this trend was not visible in the CR or the Tt results. While LD had lower CR than all Zr groups, there were no differences in Tt%. CR and Tt tests showed no differences among Zr groups. Comparing individual brands showed that none of the tests correlated with each other in the rankings of brands for both strength and translucency.
Conclusions: Zr is stronger than LD. SHT Zr is significantly weaker than other classes of Zr. TP results show increasing transparency from FS to SHT Zr. However, there are large variances in strength and transparency from brand to brand that are also dependent on the test used. Currently fracture toughness results are being collated. There is a need for a more extensive and comprehensive evaluation of Zr products in the market to better clarify the differences among Zr classes.